Nitrate levels have significantly reduced at a North Canterbury stream less than two months into a pilot project to improve its water quality.

The Institute of Environmental and Scientific Research (ESR)-led denitrification wall trial at Silverstream Reserve, near Kaiapoi, has resulted in nitrate levels in groundwater dropping from 7.1mg/l to 0.5 mg/l.

The 25 metre-long wall, installed in November, is a world first, having never been tested in a fast-flowing gravel aquifer system before.

ESR senior scientist Lee Burbery said he was delighted the wall was working as he anticipated.

"The wall itself is effectively removing all of the nitrate in the groundwater that passes through it. We are seeing a plume of treated groundwater extend down-gradient from the wall."

The denitrification wall acts as a groundwater filter. Woodchip mixed with gravel removes nitrate from the groundwater that passes through the wall. Carbon from the woodchip provides a food source for bacteria that then convert the nitrate into the harmless di-nitrogen gas (N2), which makes up 80 per cent of the air we breathe.